My W203 / M271 will not fire. Engine cranks fine but the engine tries to start but will not fire (see movie file attached). It sounds to me like a spark or timing issue because once i turn the key off there is the slightest cough from one of the cylinders. I am wondering if it is possible for the crank and cams to be out of sync by 360 degrees? I say this because this starting issue has only started after i have put the cylinder head back on the car after i had checked the valves after the timing slipped due to worn intake cam gear. While conducting the repair of the cam gears i don't remember turning the crank more than a quarter of a turn at any one time but yet i can't figure out why the engine will not start again. While lining up the cams and crank i rotated the engine a number of times and everything moved easily and there was no fouling of valves and pistons There is definitely fuel getting to the engine because when i first tried to start the engine there was the smell of fuel and i did have a fuel leak where the injector seals had not seated correctly I will check the spark plugs to be sure i have a spark but at this time of writing i can't see why the plugs should suddenly need replacing when they worked fine before i stripped down the head Is it possible that for some reason the crankshaft and camshafts can be out of sync. I'm presuming not because the crank position sensor can only be on one place of the crankshaft and even if the engine had been turned 360 degrees the sensor would still be in the same place. The same must be true for the camshaft sensor as it can only be in one place and if all of my timing marks line up, then what else could be the reason for not starting. Does anyone have an ideas? Thanks.